Turkey is assembling a contingent of thousands of Syrian Arab fighters who it insists should lead the offensive to retake Islamic State’s stronghold of Raqqa instead of the Syrian Kurdish militia the U.S. favors, according to commanders of the force.

The Arab force is central to tensions between Washington and Ankara, its North Atlantic Treaty Organization ally, over the campaign to unseat the extremist group from its de facto Syrian capital. The dispute has hampered preparations for the offensive.